Motion detecting system for use in a safety system for power equipment

ABSTRACT

A method of controlling a woodworking machine having a movable cutting tool. The method monitors a signal for a change indicative of a dangerous condition between the cutting tool and a person, senses movement of the cutting tool, and performs an action to mitigate the dangerous condition when the signal change and movement of the cutting tool are both detected. A woodworking machine including a detection system adapted to detect a dangerous condition between a person and a working portion of the machine and then to perform some action to mitigate the dangerous condition is also disclosed. A motion detection system is adapted to detect motion of the working portion and to disable the reaction system when the working portion is not moving.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/810,196, filed Jun. 4, 2007, issuing as U.S. Pat. No. 7,681,479 onMar. 23, 2010, which in turn is a continuation of U.S. patentapplication Ser. No. 09/929,234, filed Aug. 13, 2001, issued as U.S.Pat. No. 7,225,712 on Jun. 5, 2007, which claims the benefit of andpriority from the following U.S. Provisional Patent Applications: Ser.No. 60/225,056, filed Aug. 14, 2000, Ser. No. 60/225,057, filed Aug. 14,2000, Ser. No. 60/225,058, filed Aug. 14, 2000, Ser. No. 60/225,059,filed Aug. 14, 2000, Ser. No. 60/225,089, filed Aug. 14, 2000, Ser. No.60/225,094, filed Aug. 14, 2000, Ser. No. 60/225,169, filed Aug. 14,2000, Ser. No. 60/225,170, filed Aug. 14, 2000, Ser. No. 60/225,200,filed Aug. 14, 2000, Ser. No. 60/225,201, filed Aug. 14, 2000, Ser. No.60/225,206, filed Aug. 14, 2000, Ser. No. 60/225,210, filed Aug. 14,2000, Ser. No. 60/225,211, filed Aug. 14, 2000, and Ser. No. 60/225,212,filed Aug. 14, 2000. The identified patent and applications are allincorporated by reference in their entireties.

FIELD

The present invention relates to safety systems, and more particularlyto a high-speed safety system for use on power equipment.

BACKGROUND

Beginning with the industrial revolution and continuing to the present,mechanized equipment has allowed workers to produce goods with greaterspeed and less effort than possible with manually-powered tools.Unfortunately, the power and high operating speeds of mechanizedequipment creates a risk for those operating such machinery. Each yearthousands of people are maimed or killed by accidents involving powerequipment.

As might be expected, many systems have been developed to minimize therisk of injury when using power equipment. Probably the most commonsafety feature is a guard that physically blocks an operator from makingcontact with dangerous components of machinery, such as belts, shafts orblades. In many cases, guards are effective to reduce the risk ofinjury, however, there are many instances where the nature of theoperations to be performed precludes using a guard that completelyblocks access to hazardous machine parts.

Various systems have been proposed to prevent accidental injury whereguards cannot effectively be employed. For instance, U.S. Pat. Nos.941,726, 2,978,084, 3,011,610, 3,047,116, 4,195,722 and 4,321,841, thedisclosures of which are incorporated herein by reference, all disclosesafety systems for use with power presses. These systems utilize cablesattached to the wrists of the operator that either pull back a user'shands from the work zone upon operation or prevent operation until theuser's hands are outside the danger zone. U.S. Pat. Nos. 3,953,770,4,075,961, 4,470,046, 4,532,501 and 5,212,621, the disclosures of whichare incorporated herein by reference, disclose radio-frequency safetysystems which utilize radio-frequency signals to detect the presence ofa user's hand in a dangerous area of the machine and thereupon preventor interrupt operation of the machine.

U.S. Pat. Nos. 4,959,909, 5,025,175, 5,122,091, 5,198,702, 5,201,684,5,272,946, and 5,510,685 disclose safety systems for use withmeat-skinning equipment, and are incorporated herein by reference. Thesesystems interrupt or reverse power to the motor, or disengage a clutch,upon contact with a user's hand by any dangerous portion of the machine.Typically, contact between the user and the machine is detected bymonitoring for electrical contact between a fine wire mesh in a gloveworn by the user and some metal component in the dangerous area of themachine.

U.S. Pat. Nos. 3,785,230 and 4,026,177, the disclosures of which areherein incorporated by reference, disclose a safety system for use oncircular saws to stop the blade when a user's hand approaches the blade.The system uses the blade as an antenna in an electromagnetic proximitydetector to detect the approach of a user's hand prior to actual contactwith the blade. Upon detection of a user's hand, the system engages abrake using a standard solenoid. U.S. Pat. No. 4,117,752, which isherein incorporated by reference, discloses a similar braking system foruse with a band saw, where the brake is triggered by actual contactbetween the user's hand and the blade.

It is often necessary for an equipment operator to touch the blade orother cutting device of power equipment when the blade or device is notmoving (e.g., to adjust the blade, perform equipment maintenance, etc.).Thus, it would be desirable to disable the safety system when the bladeis not moving since there is no danger to the user from contact with theblade.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a machine with a fast-actingsafety system according to the present invention.

FIG. 2 is a schematic diagram of an exemplary safety system in thecontext of a machine having a circular blade.

FIG. 3 is a partial cross-section view of an exemplary magnetic sensor,assembly according to the present invention, where the arbor is not incross-sectional view.

FIG. 4 is a schematic diagram of an exemplary circuit according to thepresent invention for use with a magnetic sensor assembly.

FIG. 5 is a schematic view of an exemplary EMF sensor assembly accordingto the present invention.

FIG. 6 is a partial cross-section view of an exemplary optical sensorassembly according to the present invention, where the arbor is not incross-sectional view.

FIG. 7 is a side elevation of an alternative optical sensor assemblyaccording to the present invention.

FIG. 8 is a cross-section view of the alternative optical sensorassembly of FIG. 7, taken generally along the line 8-8.

FIG. 9. is a schematic diagram of an exemplary circuit according to thepresent invention for use with an optical sensor assembly.

FIG. 10 is a partial cross-section view of an exemplary electricalsensor assembly according to the present invention, where the arbor isnot in cross-sectional view.

FIG. 11 is a schematic side elevation of an alternative electricalsensor assembly according to the present invention.

DETAILED DESCRIPTION

A machine is shown schematically in FIG. 1 and indicated generally at10. Machine 10 may be any of a variety of different machines adapted forcutting workpieces, such as wood, including a table saw, miter saw (chopsaw), radial arm saw, circular saw, band saw, jointer, planer, etc.Machine 10 includes an operative structure 12 having a cutting tool 14and a motor assembly 16 adapted to drive the cutting tool. Machine 10also includes a safety system 18 configured to minimize the potential ofa serious injury to a person using machine 10. Safety system 18 isadapted to detect the occurrence of one or more dangerous conditionsduring use of machine 10. If such a dangerous condition is detected,safety system 18 is adapted to engage operative structure 12 to limitany injury to the user caused by the dangerous condition.

Machine 10 also includes a suitable power source 20 to provide power tooperative structure 12 and safety system 18. Power source 20 may be anexternal power source such as line current, or an internal power sourcesuch as a battery. Alternatively, power source 20 may include acombination of both external and internal power sources. Furthermore,power source 20 may include two or more separate power sources, eachadapted to power different portions of machine 10.

It will be appreciated that operative structure 12 may take any one ofmany different forms, depending on the type of machine 10. For example,operative structure 12 may include a stationary housing configured tosupport motor assembly 16 in driving engagement with cutting tool 14.Alternatively, operative structure 12 may include a movable structureconfigured to carry cutting tool 14 between multiple operatingpositions. As a further alternative, operative structure 12 may includeone or more transport mechanisms adapted to convey a workpiece towardand/or away from cutting tool 14.

Motor assembly 16 includes one or more motors adapted to drive cuttingtool 14. The motors may be either directly or indirectly coupled to thecutting tool, and may also be adapted to drive workpiece transportmechanisms. Cutting tool 14 typically includes one or more blades orother suitable cutting implements that are adapted to cut or removeportions from the workpieces. The particular form of cutting tool 14will vary depending upon the various embodiments of machine 10. Forexample, in table saws, miter saws, circular saws and radial arm saws,cutting tool 14 will typically include one or more circular rotatingblades having a plurality of teeth disposed along the perimetrical edgeof the blade. For a jointer or planer, the cutting tool typicallyincludes a plurality of radially spaced-apart blades. For a band saw,the cutting tool includes an elongate, circuitous tooth-edged band.

Safety system 18 includes a detection subsystem 22, a reaction subsystem24 and a control subsystem 26. Control subsystem 26 may be adapted toreceive inputs from a variety of sources including detection subsystem22, reaction subsystem 24, operative structure 12 and motor assembly 16.The control subsystem may also include one or more sensors adapted tomonitor selected parameters of machine 10. In addition, controlsubsystem 26 typically includes one or more instruments operable by auser to control the machine. The control subsystem is configured tocontrol machine 10 in response to the inputs it receives.

Detection subsystem 22 is configured to detect one or more dangerous, ortriggering, conditions during use of machine 10. For example, thedetection subsystem may be configured to detect that a portion of theuser's body is dangerously close to, or in contact with, a portion ofcutting tool 14. As another example, the detection subsystem may beconfigured to detect the rapid movement of a workpiece due to kickbackby the cutting tool, as is described in U.S. Provisional PatentApplication Ser. No. 60/182,866, the disclosure of which is hereinincorporated by reference. In some embodiments, detection subsystem 22may inform control subsystem 26 of the dangerous condition, which thenactivates reaction subsystem 24. In other embodiments, the detectionsubsystem may be adapted to activate the reaction subsystem directly.

Once activated in response to a dangerous condition, reaction subsystem24 is configured to engage operative structure 12 quickly to preventserious injury to the user. It will be appreciated that the particularaction to be taken by reaction subsystem 24 will vary depending on thetype of machine 10 and/or the dangerous condition that is detected. Forexample, reaction subsystem 24 may be configured to do one or more ofthe following: stop the movement of cutting tool 14, disconnect motorassembly 16 from power source 20, place a barrier between the cuttingtool and the user, or retract the cutting tool from its operatingposition, etc. The reaction subsystem may be configured to take acombination of steps to protect the user from serious injury. Placementof a barrier between the cutting tool and teeth is described in moredetail in U.S. Provisional Patent Application Ser. No. 60/225,206,entitled “Cutting Tool Safety System,” filed Aug. 14, 2000 by SD3, LLC,the disclosure of which is herein incorporated by reference. Retractionof the cutting tool from its operating position is described in moredetail in U.S. Provisional Patent Application Ser. No. 60/225,089,entitled “Retraction System For Use In Power Equipment,” filed Aug. 14,2000 by SD3, LLC, the disclosure of which is herein incorporated byreference.

The configuration of reaction subsystem 24 typically will vary dependingon which action(s) are taken. In the exemplary embodiment depicted inFIG. 1, reaction subsystem 24 is configured to stop the movement ofcutting tool 14 and includes a brake mechanism 28, a biasing mechanism30, a restraining mechanism 32, and a release mechanism 34. Brakemechanism 28 is adapted to engage operative structure 12 under theurging of biasing mechanism 30. During normal operation of machine 10,restraining mechanism 32 holds the brake mechanism out of engagementwith the operative structure. However, upon receipt of an activationsignal by reaction subsystem 24, the brake mechanism is released fromthe restraining mechanism by release mechanism 34, whereupon, the brakemechanism quickly engages at least a portion of the operative structureto bring the cutting tool to a stop.

It will be appreciated by those of skill in the art that the exemplaryembodiment depicted in FIG. 1 and described above may be implemented ina variety of ways depending on the type and configuration of operativestructure 12. Turning attention to FIG. 2, one example of the manypossible implementations of safety system 18 is shown. System 18 isconfigured to engage an operative structure having a cutting tool in theform of a circular blade 40 mounted on a rotating shaft or arbor 42.Blade 40 includes a plurality of cutting teeth (not shown) disposedaround the outer edge of the blade. As described in more detail below,braking mechanism 28 is adapted to engage the teeth of blade 40 and stopthe rotation of the blade. U.S. Provisional Patent Application Ser. No.60/225,210, entitled “Translation Stop For Use In Power Equipment,”filed Aug. 14, 2000 by SD3, LLC, the disclosure of which is hereinincorporated by reference, describes other systems for stopping themovement of the cutting tool. U.S. Provisional Patent Application Ser.No. 60/225,058, entitled “Table Saw With Improved Safety System,” filedAug. 14, 2000 by SD3, LLC, and U.S. Provisional Patent Application Ser.No. 60/225,057, entitled “Miter Saw With Improved Safety System,” filedAug. 14, 2000 by SD3, LLC, the disclosures of which are hereinincorporated by reference, describe safety system 18 in the context ofparticular types of machines 10.

In the exemplary implementation, detection subsystem 22 is adapted todetect the dangerous condition of the user coming into contact withblade 40. The detection subsystem includes a sensor assembly, such ascontact detection plates 44 and 46, capacitively coupled to blade 40 todetect any contact between the user's body and the blade. Typically, theblade, or some larger portion of cutting tool 14 is electricallyisolated from the remainder of machine 10. Alternatively, detectionsubsystem 22 may include a different sensor assembly configured todetect contact in other ways, such as optically, resistively, etc. Inany event, the detection subsystem is adapted to transmit a signal tocontrol subsystem 26 when contact between the user and the blade isdetected. Various exemplary embodiments and implementations of detectionsubsystem 22 are described in more detail in U.S. Provisional PatentApplication Ser. No. 60/225,200, entitled “Contact Detection System ForPower Equipment,” filed Aug. 14, 2000 by SD3, LLC, and U.S. ProvisionalPatent Application Ser. No. 60/225,211, entitled “Apparatus And MethodFor Detecting Dangerous Conditions In Power Equipment,” filed Aug. 14,2000 by SD3, LLC, the disclosures of which are herein incorporated byreference.

Control subsystem 26 includes one or more instruments 48 that areoperable by a user to control the motion of blade 40. Instruments 48 mayinclude start/stop switches, speed controls, direction controls, etc.Control subsystem 26 also includes a logic controller 50 connected toreceive the user's inputs via instruments 48. Logic controller 50 isalso connected to receive a contact detection signal from detectionsubsystem 22. Further, the logic controller may be configured to receiveinputs from other sources (not shown) such as blade motion sensors,workpiece sensors, etc. In any event, the logic controller is configuredto control operative structure 12 in response to the user's inputsthrough instruments 48. However, upon receipt of a contact detectionsignal from detection subsystem 22, the logic controller overrides thecontrol inputs from the user and activates reaction subsystem 24 to stopthe motion of the blade. Various exemplary embodiments andimplementations of control subsystem 26 are described in more detail inU.S. Provisional Patent Application Ser. No. 60/225,059, entitled “LogicControl For Fast Acting Safety System,” filed Aug. 14, 2000 by SD3, LLC,the disclosure of which is herein incorporated by reference.

In the exemplary implementation, brake mechanism 28 includes a pawl 60mounted adjacent the edge of blade 40 and selectively moveable to engageand grip the teeth of the blade. Pawl 60 may be constructed of anysuitable material adapted to engage and stop the blade. As one example,the pawl may be constructed of a relatively high strength thermoplasticmaterial such as polycarbonate, ultrahigh molecular weight polyethylene(UHMW) or Acrylonitrile Butadiene Styrene (ABS), etc., or a metal suchas aluminum, etc. It will be appreciated that the construction of pawl60 will vary depending on the configuration of blade 40. In any event,the pawl is urged into the blade by a biasing mechanism in the form of aspring 66. In the illustrative embodiment shown in FIG. 2, pawl 60 ispivoted into the teeth of blade 40. It should be understood that slidingor rotary movement of pawl 60 may also be used. The spring is adapted tourge pawl 60 into the teeth of the blade with sufficient force to gripthe blade and quickly bring it to a stop.

The pawl is held away from the edge of the blade by a restrainingmechanism in the form of a fusible member 70. The fusible member isconstructed of a suitable material adapted to restrain the pawl againstthe bias of spring 66, and also adapted to melt under a determinedelectrical current density. Examples of suitable materials for fusiblemember 70 include NiChrome wire, stainless steel wire, etc. The fusiblemember is connected between the pawl and a contact mount 72. Preferably,fusible member 70 holds the pawl relatively close to the edge of theblade to reduce the distance the pawl must travel to engage the blade.Positioning the pawl relatively close to the edge of the blade reducesthe time required for the pawl to engage and stop the blade. Typically,the pawl is held approximately 1/32-inch to ¼-inch from the edge of theblade by fusible member 70, however other pawl-to-blade spacings mayalso be used within the scope of the invention.

Pawl 60 is released from its unactuated, or cocked, position to engageblade 40 by a release mechanism in the form of a firing subsystem 76.The firing subsystem is coupled to contact mount 72, and is configuredto melt fusible member 70 by passing a surge of electrical currentthrough the fusible member. Firing subsystem 76 is coupled to logiccontroller 50 and activated by a signal from the logic controller. Whenthe logic controller receives a contact detection signal from detectionsubsystem 22, the logic controller sends an activation signal to firingsubsystem 76, which melts fusible member 70, thereby releasing the pawlto stop the blade. Various exemplary embodiments and implementations ofreaction subsystem 24 are described in more detail in U.S. ProvisionalPatent Application Ser. No. 60/225,056, entitled “Firing Subsystem ForUse In Fast Acting Safety System,” filed Aug. 14, 2000 by SD3, LLC, U.S.Provisional Patent Application Ser. No. 60/225,170, entitled“Spring-Biased Brake Mechanism for Power Equipment,” filed Aug. 14, 2000by SD3, LLC, and U.S. Provisional Patent Application Ser. No.60/225,169, entitled “Brake Mechanism For Power Equipment,” filed Aug.14, 2000 by SD3, LLC, the disclosures of which are herein incorporatedby reference.

It will be appreciated that activation of the brake mechanism willrequire the replacement of one or more portions of safety system 18. Forexample, pawl 60 and fusible member 70 typically must be replaced beforethe safety system is ready to be used again. Thus, it may be desirableto construct one or more portions of safety system 18 in a cartridgethat can be easily replaced. For example, in the exemplaryimplementation depicted in FIG. 2, safety system 18 includes areplaceable cartridge 80 having a housing 82. Pawl 60, spring 66,fusible member 70 and contact mount 72 are all mounted within housing82. Alternatively, other portions of safety system 18 may be mountedwithin the housing. In any event, after the reaction system has beenactivated, the safety system can be reset by replacing cartridge 80. Theportions of safety system 18 not mounted within the cartridge may bereplaced separately or reused as appropriate. Various exemplaryembodiments and implementations of a safety system using a replaceablecartridge are described in more detail in U.S. Provisional PatentApplication Ser. No. 60/225,201, entitled “Replaceable Brake MechanismFor Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, and U.S.Provisional Patent Application Ser. No. 60/225,212, entitled “BrakePositioning System,” filed Aug. 14, 2000 by SD3, LLC, the disclosures ofwhich are herein incorporated by reference.

While one particular implementation of safety system 18 has beendescribed, it will be appreciated that many variations and modificationsare possible within the scope of the invention. Many such variations andmodifications are described in U.S. Provisional Patent Application Ser.No. 60/182,866, filed Feb. 16, 2000 and U.S. Provisional PatentApplication Ser. No. 60/157,340, filed Oct. 1, 1999, the disclosures ofwhich are herein incorporated by reference.

As mentioned above, safety system 18 may include a sensor or sensorassembly for detecting motion of the blade or cutting tool. The sensorassembly typically is coupled to send a signal to logic controller 50indicating whether the blade is in motion. The logic controller may beconfigured to respond differently to the detection of a dangerouscondition based on whether the blade is moving. For example, it is oftennecessary for a user of machine 10 to touch blade 40 when preparing themachine for use, and when installing or removing the blade. Usually, theuser would disconnect all power from machine 10 while performing suchoperations. However, in the event that the user neglects to disconnectthe machine from power source 20 before touching the blade, logiccontroller 50 would receive a contact detection signal from detectionsubsystem 22. If safety system 18 includes a blade motion sensor, thenlogic controller 50 may be configured not to actuate firing subsystem 76when the blade is not moving. Instead, the logic controller may beconfigured to take one or more other actions such as disabling motorassembly 16, sounding an alarm, displaying an error, etc. Alternatively,the logic controller may be configured to take no action if contact isdetected while the blade is not moving.

In addition to detecting whether the blade is moving, safety system 18may also be configured to determine the speed at which the blade ismoving. This allows the logic controller to distinguish between rapidblade movement which could cause injury to the user, and slow blademovement which generally would not cause injury to the user. Thus, forexample, a user could move the blade by hand without actuating firingsubsystem 76. In some embodiments, the blade motion sensor may beconfigured to determine relative blade speed. In alternativeembodiments, logic controller 50 may be configured to analyze the signalfrom the blade motion sensor to determine relative blade speed.

It will be appreciated that the speed at which a blade is consideredlikely to cause injury will vary depending on the type of machine 10 andblade 40. For example, a 14-inch carbide tooth blade on a table saw willcause serious injury at a lower speed than a 5⅜-inch plywood blade on acordless trim saw. Thus, an embodiment of safety system 18 for use onthe table saw may be configured to actuate the firing subsystem only atblade speeds above approximately 10, 25, 60, or 90 rpm, while analternative embodiment of safety system 18 for use on the trim saw maybe configured to actuate the firing subsystem only at blade speeds aboveapproximately 40, 100, or 240 rpm.

Alternatively or additionally, the logic controller may be configured tointerpret blade motion as being dangerous only when detected during orsoon after motor assembly 16 was in operation. In other words, the blademotion detection would only be active while the blade was being moved bythe motor assembly and during a relatively brief period afterward whilethe blade was coasting to a stop. Any blade motion detected at othertimes would be ignored.

Safety system 18 may include any of a wide variety of sensor assembliesto detect blade movement. Furthermore, each sensor assembly may beadapted as necessary depending on the particular type of blade 40 and/orthe configuration of machine 10. While several exemplary sensorassemblies are described herein, it will be understood that all methodsand mechanisms suitable for automatically detecting the motion of ablade are within the scope of the invention.

One exemplary embodiment of safety system 18 includes a magnetic sensorassembly 1000 configured to detect movement of the blade. It will beappreciated that the blade movement may be detected by monitoring theblade or any other portion of the safety system that moves with theblade, including the arbor, bearings, motor assembly, arbor pulley, etc.In the exemplary implementation depicted in FIG. 3, magnetic sensorassembly 1000 includes a Hall effect sensor 1001 and one or more magnets1002. A coil could also be used to detect magnetic field fluctuationsfrom rotation. The magnets are mounted on arbor 42. Sensor 1001 ismounted and configured to detect blade motion by detecting the movementof the magnets on the arbor. Sensor 1001 may be any suitable Hall effectsensor such as, for example, the sensor available from MicronasIntermetall of San Jose, Calif., under the part no. HAL 114.

Hall effect sensor 1001 may be mounted adjacent the arbor by anysuitable method. In the exemplary implementation, the sensor is mountedin a recessed region 272 of an insulating tube 268. The insulating tubealso supports charge plates 44 and 46, as is described in more detail inU.S. Provisional Application Ser. No. 60/225,211, entitled “Apparatusand Method for Detecting Dangerous Conditions in Power Equipment,” filedAug. 14, 2000, by SD3, LLC. The recessed region is disposed at leastpartially over a hole 273 in charge plate 44. Alternatively the recessedregion may be disposed over a hole 273 in charge plate 46. In any event,magnet 1002 is disposed on arbor 42 to pass beneath or adjacent hole 273as the arbor rotates within the insulating tube. Hole 273 allows sensor1001 to detect the field created by magnet 1002 as it passes. Sensor1001 includes one or more connector leads 1003 connectable to receivepower from, and transmit signals to, logic controller 50.

Magnets 1002 may be mounted on the arbor in any suitable fashion.Typically, the magnets are mounted so as not to extend above the surfaceof the arbor. For example, the magnets may be press-fit and/or glued ina recess formed on the arbor. Alternatively, one or more of the magnetsmay be mounted to extend above the surface of the arbor. The size andnumber of magnets 1002 may be varied to control the signal produced bysensor 1001. In alternative embodiments, magnets 1002 may be mounted atother locations such as an end of arbor 42, on blade 40, etc.

Sensor 1001 may be connected to send signals to logic controller 50 viaany suitable circuitry. For example, FIG. 4 illustrates one exemplaryrotation sense circuit 177 adapted to couple the signals from sensor1001 to logic controller 50. Those of skill in the art will appreciatethat circuit 177 may be modified as needed for a particular application.

Another example of a suitable method for detecting blade motion isthrough electromagnetic field (EMF) measurements. As is known to thoseof skill in the art, when power to an electric motor is shut off, themotor will produce EMF pulses on the input power cables as the motorspins down. Thus, where blade 40 is driven by an electric motor assembly16, the blade may be assumed to be in motion whenever an EMF pulse isdetected on the power supply cables, as well as whenever power is beingsupplied to the motor assembly.

Thus, in another exemplary embodiment depicted in FIG. 5, safety system18 includes an EMF sensor assembly 1005 configured to detect motion ofblade 40. Sensor assembly 1005 includes an EMF detection circuit 1006disposed in the power supply path between motor assembly 16 and powersource 20. Circuit 1006 is adapted to monitor power cables 1007 whichextend between the power source and the motor assembly, and to detectthe presence of EMF pulses on the cables. Alternatively, circuit 1006may be disposed at any other location suitable for detecting EMF pulsesfrom motor assembly 16. Circuit 1006 may be any circuit or mechanismadapted to detect EMF pulses, such as are known to those of skill in theart. Circuit 1006 is also coupled to logic controller 50, and adapted toconvey a signal to the logic controller indicating the presence and/orabsence of EMF pulses on cables 1007. Optionally, circuit 1006 and/orlogic controller 50 may be adapted to analyze the detected EMFemissions, and evaluate the speed of blade 40. In such case, the logiccontroller may be configured not to actuate firing subsystem 76 when thespeed of the blade is unlikely to cause serious injury to the user.

In another exemplary embodiment, safety system 18 includes an opticalsensor assembly adapted to optically detect movement of blade 40. Safetysystem 18 may be configured to optically detect blade motion in avariety of ways. For example, a rotary optical encoder may be coupled tothe arbor to detect rotation of the arbor. Any rotary encoder may beused, such as those available from Omron Electronics Inc., ofSchaumburg, Ill. Alternatively, other optical sensor assemblies may beused as described below.

Typically, the optical sensor assembly will be at least partiallyenclosed to prevent saw dust or other debris from interfering with thedetection. One exemplary implementation of an optical sensor assembly isindicated generally at 1010 in FIG. 6. Sensor assembly 1010 includes anoptical detector 1011 adapted to detect light from an optical source1012. Alternatively, plural optical sources and/or plural opticaldetectors may be used. It will be appreciated that any of a variety ofdifferent optical sources may be used which are known to those of skillin the art, including an incandescent or fluorescent bulb, lightemitting diode (LED), laser diode, etc. Similarly, any of a variety ofdifferent optical detectors may be used which are known to those ofskill in the art, including a photodiode, phototransistor, etc.

In any event, the optical source is arranged so that the signal receivedat the optical detector when the blade is moving is different than thesignal received when the blade is stationary. For example, the sourceand detector may be arranged so that a signal is received only when theblade is moving, or only when the blade is stationary. Alternatively,source 1012 and detector 1011 may be arranged so that the amount ofemitted light that reaches the detector varies when the blade is inmotion.

The implementation depicted in FIG. 6 uses this latter arrangement.Sensor assembly 1010 includes an LED 1012 mounted in insulating tube 268to emit light through hole 273 in charge plate 44 or 46. The lightreflects off arbor 42 and is detected by a photodiode 1011 which is alsomounted in insulating tube 268 adjacent hole 273. The arbor includes oneor more reduced-reflection regions 1013 adapted to reduce the amount oflight reflected to photodiode 1011. Regions 1013 may be formed bycoating the arbor with a light-absorbing coating, roughening the arborto cause random scattering of the light, etc. In any event, the reducedreflecting regions create a varying signal at the photodiode when thearbor is rotating. In contrast, a constant signal is produced at thephotodiode when the arbor is stationary.

The minimal clearance between arbor 42 and charge plates 44, 46 tends tomaintain the space between the arbor and the photodiode/LED relativelyfree of debris which could block the signal. Alternatively, theinsulating tube assembly may be sealed in a protective housing (notshown).

In another alternative implementation depicted in FIGS. 7 and 8, opticalsensor assembly 1010 includes a barrier member 1014 mounted on the arborand disposed between photodiode 1011 and LED 1012. Alternatively, thebarrier member may be mounted on any other portion of cutting tool 14 ormotor assembly 16 adapted to move with the blade. Barrier member 1014includes one or more light-transmitting regions or holes 1015, which maytake any desired shape or size. The photodiode and LED are mounted in asupport member 1016 attached to an arbor block 250, and disposed oneither side of barrier member 1014. The photodiode is aligned so thatemitted light will pass through holes 1015. Likewise, the LED is alignedto detect the light which passes through the holes. Thus, as arbor 42rotates, light from the LED is alternately blocked and transmitted bythe barrier member, thereby creating a varying signal at the photodiode.

Photodiode 1011 and LED 1012 may be connected to any suitable drivingcircuitry such as are known to those of skill in the art. FIG. 9 showsone exemplary circuitry for producing an optical signal at LED 1012 anddetecting the signal at photodiode 1011. The particular values of thecircuit components and voltage supplies may be selected as desired for aspecific application. In any event, the photodiode is coupled totransmit a signal to logic controller 50 to indicate whether blade 40 ismoving.

In another exemplary embodiment, safety system 18 includes an electricalsensor assembly adapted to electrically detect movement of blade 40.There are numerous methods and mechanisms for electrically detectingblade movement within the scope of the invention. The particular methodand/or mechanism selected will typically depend on the specific type andconfiguration of machine 10. For example, where charge plate 46 isconfigured to capacitively detect a signal induced in the blade, anyincidental eccentricity in the blade or the blade rotation will causethe capacitance between the blade and charge plate 46 to vary as theblade rotates. As a result, charge plate 46 will detect a varying signalamplitude when the blade is rotating. Thus, a single sensor may beconfigured to detect both contact with the user and rotation of theblade. Preferably, the incidental variation fluctuation is insufficientin magnitude and/or rate of change to trigger reaction subsystem 24.

Rather than rely on incidental eccentricities, safety system 18 mayinclude an exemplary electrical sensor assembly adapted to detect asignal variation caused by a designed eccentricity or non-uniformity inthe blade. Alternatively, the sensor assembly may be adapted to detectthe signal from an eccentricity in some portion of cutting tool 14 thatmoves with the blade and is electrically coupled to the blade. Oneexemplary implementation of such a sensor assembly is indicatedgenerally at 1020 in FIG. 10. Sensor assembly 1020 includes a detectionelectrode 1021 capacitively coupled to detect an electrical signal onarbor 42. Electrode 1021 may be mounted in any suitable fashion toprovide electrical insulation from arbor 42 as well as the remainder ofcutting tool 14 and machine 10. In the exemplary implementation,electrode 1021 is mounted in insulating tube 268 and arranged to extendto a point closely adjacent the arbor between charge plates 44 and 46.Sensor assembly 1020 also includes one or more eccentricities 1022disposed on the arbor and substantially aligned with electrode 1021 soas to pass by the electrode as the arbor rotates.

It will be appreciated that eccentricities 1022 may be configured in anydesired quantity, size, shape or form adapted to cause a variation inthe capacitance between the arbor and the electrode as the arborrotates. In the exemplary implementation, eccentricities 1022 take theform of beveled regions formed on the surface of arbor 42. Thus, thespace between the electrode and the arbor is greater (and therefore thecapacitance is less) when an eccentricity is positioned beneath theelectrode than when an eccentricity is not positioned beneath theelectrode. Alternatively, eccentricities 1022 may take other formsadapted to vary the capacitance between the arbor and electrode,including raised regions, dielectric pads, etc. In any event, if anelectrical signal is induced in the arbor (e.g., by charge plate 44 ofcontact detection subsystem 22), then electrode 1021 will detectvariations in that signal if the arbor is rotating. Conversely, theelectrode will detect no variations in the signal if the arbor isstationary.

Turning attention now to FIG. 11, another exemplary implementation ofelectrical sensor assembly 1020 is shown in which electrode 1021 isdisposed adjacent the teeth 1023 of blade 40. Electrode 1021 may bemounted on arbor block 250 or any other suitable portion of machine 10.Additionally, the electrode may be positioned at the side of the blade(as shown in FIG. 11) or at the perimeter of the blade facing in towardthe arbor. The size, shape and position of the electrode may varydepending on the position and size of teeth 1023. In any event, as teeth1023 pass by electrode 1021, the capacitance between the blade and theelectrode varies, thereby varying the amplitude of the signal detectedby the electrode. Alternatively, a plurality of electrodes may bepositioned at various points adjacent the teeth so that blade motionwould be detected by modulations in the relative signal amplitudes atthe electrodes. Such an alternative detection mechanism may also be usedwith other implementations of sensor assembly 1020.

While a few exemplary magnetic, EMF, optical and electrical sensorassemblies have been described for detecting blade motion, it will beappreciated that many modifications and variations to such sensorassemblies are included within the scope of the invention. Furthermore,safety system 18 may include other types of motion detection sensorssuch as mechanical sensors, sonic and ultra-sonic sensors, etc. In anyevent, the invention provides effective and reliable means fordiscriminating between conditions which are, and are not, likely tocause injury to a user of power machinery.

It is believed that the disclosure set forth above encompasses multipledistinct inventions with independent utility. While each of theseinventions has been disclosed in its preferred form, the specificembodiments thereof as disclosed and illustrated herein are not to beconsidered in a limiting sense as numerous variations are possible. Thesubject matter of the inventions includes all novel and non-obviouscombinations and subcombinations of the various elements, features,functions and/or properties disclosed herein. No single feature,function, element or property of the disclosed embodiments is essentialto all of the disclosed inventions. Similarly, where the claims recite“a” or “a first” element or the equivalent thereof, such claims shouldbe understood to include incorporation of one or more such elements,neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certaincombinations and subcombinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and subcombinations of features, functions,elements and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

1. A method of controlling a woodworking machine having a movablecutting tool, the method comprising: imparting an electric signal to apredetermined portion of the machine; monitoring the electric signal forat least one change indicative of a dangerous condition between thecutting tool and a person; sensing movement of the cutting tool; andperforming a predetermined action to mitigate the dangerous conditionwhen the change in the electric signal and movement of the cutting toolare both detected.
 2. The method of claim 1, where the predeterminedportion of the machine is the cutting tool.
 3. The method of claim 1,where the predetermined action is stopping the motion of the cuttingtool.
 4. The method of claim 1, where the predetermined action isretracting the cutting tool.
 5. The method of claim 1, where thewoodworking machine includes a motor to move the cutting tool, and wherethe predetermined action is disconnecting the motor from power.
 6. Themethod of claim 1, where the predetermined action is placing a barrieradjacent the cutting tool.
 7. The method of claim 1, where the dangerouscondition is contact with the cutting tool.
 8. The method of claim 1,where the dangerous condition is proximity to the cutting tool.
 9. Themethod of claim 1, where the woodworking machine includes a rotatablearbor, where the cutting tool is mounted on the arbor, and wheremovement of the cutting tool is sensed by sensing rotation of the arbor.10. The method of claim 1, where movement of the cutting tool is notsensed if the cutting tool is moving below a threshold speed.
 11. Themethod of claim 1, where the woodworking machine is a table saw and thecutting tool is a circular blade.
 12. The method of claim 1, wheresensing movement of the cutting tool includes application of data to analgorithm.
 13. The method of claim 12, where the application of data toan algorithm is done by a signal processor.
 14. The method of claim 1,where movement of the cutting tool is sensed through an electricalsignal.
 15. The method of claim 1, where movement of the cutting tool issensed through an optical signal.
 16. The method of claim 1, wheremovement of the cutting tool is sensed through an electromagnetic fieldsensor.
 17. A method of controlling a woodworking machine having amovable cutting tool, the method comprising: imparting an electricsignal to a predetermined portion of the machine; monitoring theelectric signal for at least one change indicative of a dangerouscondition between the cutting tool and a person; a step for sensingmovement of the cutting tool; and mitigating the dangerous conditionwhen the change in the electric signal and movement of the cutting toolare both detected.
 18. A woodworking machine comprising: a workingportion adapted to work when moving; a detection system configured todetect a dangerous condition between a person and the working portion bymonitoring an electric signal for at least one change indicative of thedangerous condition; a reaction mechanism configured to perform apredetermined action relative to the working portion upon detection ofthe dangerous condition; and means for detecting motion of the workingportion and for disabling the reaction mechanism when the workingportion is not moving.
 19. The woodworking machine of claim 18, wherethe change indicative of the dangerous condition is not dependent onmovement of the working portion.
 20. The woodworking machine of claim18, where the detection system is further configured to impart theelectric signal to the cutting tool.